Further support for this hypothesis is provided by studies demonstrating that serum levels of pro-inflammatory cytokines such as IL-1β, IL-6 and TNFα are elevated in depressed patients (Dowlati et al., 2010; Howren et al., 2009). It is well known that these cytokines can induce somatic symptoms, referred to as sickness behavior, including fatigue and loss of appetite (Schiepers et al., 2005), which overlap with common symptoms of major depression. Further, cytokines can affect mood, causing dysphoria and anxiety (Schiepers et al., 2005). Here, we explore the possibility that repeated psychological stress can induce a chronic immune reaction and elevation of cytokines and discuss how this could contribute to depression and comorbid immune/inflammation illnesses.
